Project Management guide · Cost and resources
Effort Estimation in Project Management: Methods and Examples
Effort estimation predicts how much work, measured in person-hours or person-days, a task or project needs. Effort is the basis for both the schedule (how long it will take) and the budget (how much it will cost).
Key takeaways
- Effort is work in person-hours; duration and cost are derived from it.
- Use three-point estimates when tasks are uncertain.
- Plan with realistic productive hours, not a full working day.
- Compare actuals with estimates to calibrate future estimates.
Effort vs duration vs cost
| Measure | Unit | Example |
|---|---|---|
| Effort | Person-hours or person-days | 120 person-hours of development |
| Duration | Working days or weeks | 10 working days |
| Elapsed time | Calendar days | 14 calendar days, including a weekend |
| Cost | Money | 120 hours x $60 = $7,200 |
Duration depends on effort and on how many people work on the task and how many productive hours they have each day.
Common techniques
- Expert judgment: experienced people estimate from what they have seen before.
- Analogous: compare with similar past tasks and adjust.
- Parametric: apply a productivity rate, such as hours per test case or per report.
- Three-point / PERT: (O + 4M + P) / 6 to reflect uncertainty.
- Wideband Delphi: a group estimates anonymously, discusses differences and re-estimates.
- Relative estimation: Agile teams use story points or T-shirt sizes rather than hours.
How to estimate effort step by step
- Break the work into tasks small enough to estimate, usually 8 to 80 hours each.
- Clarify what done means for each task, including testing and review.
- Ask the people who will do the work to estimate it, using three points where uncertainty is high.
- Adjust for skill level, unfamiliar technology and known risks.
- Add time for project management, meetings and rework only once, at an agreed level.
- Convert effort to duration using realistic availability, then to cost using rates.
- Record the estimate and compare it with actuals later to improve.
Worked example
A data migration is estimated at optimistic 80, most likely 110 and pessimistic 170 person-hours.
- PERT effort = (80 + 4 x 110 + 170) / 6 = 690 / 6 = 115 person-hours.
- Standard deviation = (170 - 80) / 6 = 15 hours, so roughly 100 to 130 hours is a one-standard-deviation range.
- Two analysts each have 6 productive hours a day, giving 12 hours per day. Duration = 115 / 12 = about 9.6, so 10 working days.
- At a blended rate of $60 per hour, cost = 115 x 60 = $6,900.
Common mistakes
- Assuming people are productive for a full 8-hour day.
- Confusing effort with duration and promising dates too early.
- Assuming adding people shortens a task in proportion; coordination adds effort.
- Forgetting testing, documentation, review and handover.
- Treating a single-point estimate as a commitment.
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Frequently asked questions
What is effort estimation?
Predicting the amount of work, in person-hours or person-days, needed to complete a task or project.
What is the difference between effort and duration in effort estimation?
Effort is the amount of work; duration is how long it takes in working time. Forty hours of effort can be one week for one person or half a week for two.
How do you convert an effort estimate to duration?
Divide effort by the number of people multiplied by their productive hours per day, then allow for non-working days.
What units are used for an effort estimate?
Usually person-hours, person-days or person-months. Agile teams often use story points instead.
What is the PERT formula for effort estimation?
Expected effort = (optimistic + 4 x most likely + pessimistic) / 6, with standard deviation (pessimistic - optimistic) / 6.
How many productive hours a day should effort estimation assume?
It varies by organisation; many planners assume around 5 to 6 productive hours rather than the full working day. Measure it for your own team.
What is Wideband Delphi effort estimation?
A group technique where experts estimate anonymously, discuss the reasons for differences and repeat until the estimates converge.
Who should do effort estimation?
The people who will do the work, supported by the project manager and historical data.
How is an effort estimate used in the budget?
Labour cost is effort multiplied by the hourly or daily rate of the people doing the work.
What is a person-month in effort estimation?
The work one person does in a month. Its hours depend on the convention used, so state the assumption.
Why do effort estimates go wrong?
Missing tasks, optimism, unclear requirements, interruptions and multitasking are the usual causes.